Answer:
There is no set size for a horse paddock to be, although aroun 30ftx30ft should be a decent size. My horse has a paddock smaller than this, but at my DIY livery yard, the horses have separate paddocks and don't have to share. My horse is left out while I go to school, so it is around 10 hours a day by the time I come back. Then I bring it in overnight and we also sometime rotate the paddocks to diffrent ones if the grass gets low, which it usually doesn't. So to conclude, there is no set size, but if your horse lives out 24/7 then you need at least 1 acre for one horse and then 1/2 an acre for every horse after that. Hope this helped :)